The house is locked until the Reverend reveals the location of the hideout to Geralt. The Reverend tells him that Innkeeper Olaf has a key, and indeed he does, but actually the key is not required to enter the hideout, only dispatching the two Salamandra lackeys who guard the place is necessary. However, it is clear that plot-wise, Geralt should go to the Inn first and rescue Shani and then take the key from Olaf's dead body.

Barghests have a tendency to show up around this house during the night, the Beast appears occasionally, as does a ghoul from time to time.

If you proceed directly to the Salamandra hideout without rescuing Shani first, you can dispatch the two lackeys guarding the hideout and then enter the hut without the key. This launches the second, more pitched battle, but then the trapdoor remains locked, so you are indeed forced to go rescue Shani first.